Currently, these presses are only used to obtain fats and oils that are produced in small quantities. The most representative examples are shea butter and cocoa butter. The presses consist of a perforated cylinder known as the ‘sieve’ (sieve press). The sieve is stabilized with rods, and the oil escapes through an opening that is between the rods of the sieve. A piston is used to raise the pressure inside the sieve, pushing a plunger down into the contents that must be pressed. During the pressing, the fat is stored in the trays inside the sieve, in special ducts.
This pressing system is mainly used to obtain natural or cold-pressed vegetable oils. Typical examples of this category are cold-pressed sesame oil, sunflower oil and safflower oil, and, increasingly, cold-pressed rapeseed oil. The devices used are called “spindle presses”, in which the shaft has the form of a continuous spindle and fits horizontally inside the press. To increase the pressure while the material is being pressed and to compensate for any pressure loss, the diameter of the spindle cover gradually narrows in the direction of travel. The spindle normally consists of several segments, with variations of pitches and shapes, allowing the press to adapt to different seeds.
Currently, open pressing is only used in the production of olive oil and is a discontinuous process. The pulp of the olive oil extends in some frames covered with filter cloths; these racks are then stacked (frame press). The process, which takes place inside this type of press, which can process about 500 kg of pulp, can last up to two hours. In the middle of the frame there is a perforated stopper that projects upwards, through which the olive oil can drain. The necessary pressure is generated by a hydraulic piston. Previously, olive oil was obtained by grinding olives in olive oil mills.